Hot deformation behavior of a Fe3Al-binary alloy in the A2 and B2-order regimes

A binary Fe₃Al alloy is investigated with respect to hot deformation behavior and microstructural as well as microtextural modifications. Applying the hot deformation simulator (WUMSI) to hot rolling conditions in the A2 and B2-order regimes in combination with data analysis, significant changes in deformation behavior are identified. These conditions are selected for performing hot rolling experiments. The differences in microstructure are investigated. On the basis of microtexture investigations by means of electron backscatter diffraction (EBSD) differences concerning orientation gradients and sub-grain structures are found. A model of combined order-related and non-order related effects is proposed explaining the observed material behavior. The information gained is the basis for the optimization of the thermomechanical treatment to produce ductile Fe₃Al sheet material.     

The stress anomaly in FeAl–Fe3Al alloys

The anomalous stress peak observed near 500–600 °C in Fe–Al alloys has now been convincingly explained using a model of hardening by immobile thermal vacancies on the lower temperature side of the peak and the loss of hardening as these vacancies become mobile at higher temperatures. The large numbers of vacancies required for such hardening are associated with compositions close to stoichiometry, i.e. 40–50%Al, raising the question of whether such a vacancy hardening model can be adopted for Fe₃Al alloys, which show a similar stress peak anomaly. Examination of data on vacancy formation over the entire range of composition, Fe–Fe₃Al–FeAl, shows that, indeed, a vacancy hardening model appears capable of explaining the stress anomaly for both FeAl and Fe₃Al.     

HREM observation of grown-in antiphase boundaries in an Fe3Al based alloy

Atomic arrangements of grown-in antiphase boundaries (APBs) in Fe₃Al based alloy were observed by means of high resolution electron microscopy. According to the continuity of Fe and Al sublattices across the APBs in different crystallographic planes, all observed APBs were identified as next-nearest neighbour types. When observed edge-on, the grown-in APBs are very thin. The comparison of the contrast of the atomic columns in the close vicinity of the APBs and the interior of the domains indicates sharp, un-relaxed grown-in domain walls.     

Optimisation of precipitation for controlling recrystallisation of wrought Fe3Al based alloys

A Fe–26Al–5Cr (at.%) single-phase (:A2/B2/D0₃) alloy and two-phase (+TiC) alloys with different amounts of TiC particles have been hot rolled at 800 °C and the kinetics of static recrystallisation have been studied. In the alloys with a high amount of TiC, needle-like TiC of more than 1 μm in length formed during cooling after homogenisation in the single-phase region and coarsened during hot rolling. The large particles cause particle stimulated nucleation (PSN) and hence accelerate recrystallisation. In order to accomplish both strengthening by precipitates and inhibition of recrystallisation that deteriorates room-temperature ductility, a thermo-mechanical treatment consisting of hot deformation with a low amount of precipitates and a subsequent heat treatment for further precipitation is proposed. This process is difficult to carry out in the (Fe–26Al–5Cr)–TiC system due to the high precipitation temperature of TiC. The precipitation temperature is significantly decreased by replacing TiC by VC or MoC.     

Effective formation energies of atomic defects in D03Fe3Al: an ab-initio study

The effective formation energies of atomic defects in D0₃---Fe₃Al are determined within the framework of a grand-canonical statistical method. The grand-canonical defect formation energies entering this method are calculated by the ab-initio mixed-basis pseudopotential method. Thermal vacancies appear on the sublattice of the Fe atoms with an effective formation energy of 1.25 eV and on the Al sublattice (about 1.4 eV). The structural defects are Al antisite atoms on the γ sublattice of the Fe atoms or Fe antisite atoms on the Al sublattice.     

Shear-induced chemical disordering in Ni3Al at different temperatures

Molecular dynamics simulations have been used to study shear-induced chemical disordering in Ni₃Al lattices at different temperatures and strain rates. Shearing determines the formation of an amorphous layer, the thickness of which increases linearly with the square root of time. The rate at which the amorphous layer grows is both shearing rate- and temperature-dependent. A linear correlation between the amorphous layer growth rate and the shear modulus is found. This suggests that mechanical properties could play a central role in shear-induced disordering processes.     

Processing of high carbon Fe3Al based intermetallic alloy

A melting procedure for air induction melting (AIM) of an Fe₃Al based intermetallic alloy Fe-15.38 wt%Al-1.1 wt%C is described. Use of an appropriate slag cover during AIM results in elimination of hydrogen gas porosity in cast AIM ingots. Criteria for slag selection and slag to metal ratio are discussed. Refining by slag-metal reactions results in significant reduction in impurity levels (S, O, N) during AIM. Consequently, low cost raw materials such as mild steel scrap and commercial aluminium were used for melting the alloy. The AIM ingot exhibited excellent tensile properties. The ductility and hot workability of the ingot may be further improved by subsequent processing through electroslag remelting. It is also argued that the presence of carbon may be necessary to get AIM castings with desirable mechanical properties.     

Grain refinement of a Fe3Al-based alloy using κ-Fe3AlC precipitate particles stimulating nucleation of recrystallization

Effects of particle distribution level on recrystallization were investigated in a Fe₃Al-based alloy containing coarse κ-Fe₃AlC precipitate particles. Volume fraction of 10–12% of rod-like κ particles with different size and interparticle spacing was introduced within the Fe₃Al matrix by changing the cooling rate from 1200 °C, which is above the precipitation temperature of the κ phase. These samples were warm rolled at 700 °C to a total reduction of 75%. Annealing of the warm rolled samples produced complete recrystallized structures. The average recrystallized grain size against interparticle spacing showed a valley-shaped curve with a minimum size of 20 μm. Orientation analyses of the warm rolled samples with high resolution EBSD method revealed that the valley shape of the curve may be explained by the particle stimulated nucleation density of recrystallization around κ particles, dependent on the particle distribution.     

Martensitic transformation of Ni64Al34Re2 shape memory alloy

As-rolled and annealed Ni₆₄Al₃₄Re₂ shape memory alloy (SMA) exhibits B2 → L1₀ (3R) martensitic transformation with M_s temperature up to about 210 °C. Experimental results indicate that the annealing temperature is the major factor that affects the M_s temperature. It is found that adding 2 at.% Re to replace Al in Ni₆₄Al₃₆ binary SMA can significantly refine the alloy's grain size and enhance the softening behavior during transformation. Meanwhile, Re has the same trend as Ni to affect the M_s temperature, but it has a less effect than Ni. The lattice constants and microstructures of NiAl-B2 phase, NiAl-L1₀ (3R) martensite and Ni₃Al-L1₂ phase are almost similar to those of Ni–Al binary SMAs.     

Microstructures and mechanical properties of Fe3Al-based Fe–Al–C alloys

In this paper results on the microstructures and mechanical properties of Fe₃Al-based Fe–Al–C alloys with strengthening precipitates of the perovskite-type κ-phase Fe₃AlC_x are presented. The alloys are prepared by vacuum induction melting and cast into Cu-moulds. The composition of the Fe₃Al matrix of the investigated Fe–Al–C alloys varies between 23 and 29 at.% Al. The ternary C-additions range from 1 to 3 at.%. The microstructures of the alloys are characterised by means of light optical microscopy (LOM). Phase identification is performed by means of X-ray diffraction (XRD). The strength of the alloys as a function of temperature is determined through compression tests. The room-temperature ductility is evaluated by tensile tests. The fracture surfaces of the tensile specimens are analysed using scanning electron microscopy (SEM).     

On the parameters affecting the formation of iron aluminides during pressure-assisted infiltration of aluminium into a preform of steel fibres

The parameters affecting the formation of iron aluminides during reactive infiltration of iron-based fibre preforms in a squeeze-casting equipment were investigated. The volume fraction of intermetallic phases was found to increase when the fibre volume fraction and size decreased. The increase of the temperatures of the die, aluminium and preform allowed avoiding premature chocking of the preforms, reducing the gradient of reaction and increasing the extent of exothermic reactions. As-infiltrated material with overall chemical composition corresponding to Fe₃Al was studied by SEM, X-ray diffraction and TEM. The unreacted iron fibres were surrounded by iron-rich FeAl phases. The single-phase grains adjacent to the boundary layer were identified as Fe₂Al₅ whereas the grains with lamellar morphology were identified as consisting of a mixture of FeAl and FeAl₂ lamellae. Heat treatment of such an as-infiltrated material resulted in the formation of a fine-grained structure composed of FeAl and Fe₃Al phases.     

In-situ precipitation of Al2O3 and κ-Fe3AlC0.5 in iron aluminides through spark plasma sintering: Microstructures and mechanical properties

Iron aluminides are ordered intermetallic alloys which offer good resistance to corrosion and sulfidation. At the same time, their Achilles' heel is low ductility at room temperature and sometimes they have poor mechanical properties. By means of mechanical alloying and spark plasma sintering (MA–SPS) it is possible to obtain bulk nanostructured iron aluminides which show high hardness and high yield stress.

In this work we present the production of nanostructured powders and their consolidation through spark plasma sintering. The inevitable use of methanol as processing control agent (PCA) leads to a supersaturation in carbon and oxygen of the milled powder and a consequent in-situ precipitation of carbides and oxides during SPS. The presence of carbides, oxides and a nanostructured matrix leads to high mechanical properties with hardness 5.20 ± 0.05 GPa and a yield stress of 1305 MPa.     

On the peritectoid formation of Ni5Al3

The peritectoid formation of Ni₅Al₃ from the two phases NiAl and Ni₃Al was studied in a Ni---Al alloy containing 66 at% Ni by means of transmission electron microscopy. The product phase does not form as a uniform layer between the two initial phases as expected and already observed in a few systems. In the system studied here there are only very few nucleation sites located at the NiAl/Ni₃Al interface. The further growth of Ni₅Al₃ takes place only into one of the initial phases which is NiAl. A strict orientation relationship between NiAl and Ni₅Al₃ was observed; the growth direction was [221]. The transformation is presumably diffusion controlled; it is very sluggish and it can be described by a nucleation and growth process. From the study presented here we conclude that the formation of Ni₅Al₃ proceeds by a micromechanism which differs from that normally assumed for peritectoid reactions.     

Crystallography and phase transformation mechanisms in TiAl-based alloys – A synthesis

The variation in cooling rates of Ti–46.8Al–1.7Cr–1.8Nb (at. %) alloy from the high-temperature α domain produces lamellar, Widmanstätten, feathery-like (α₂ + γ) structures, as well as γ-massive phase, often coexisting together. Earlier reported crystallographic and morphological details of each of these structures are compiled together and a combined view on the generation of their crystallographic related possible variants and their solid phase transformation mechanisms is proposed. Sympathetic nucleation is suggested as a common mechanism for the lamellar structure at slow cooling rate, the Widmanstätten structure and the feathery-like structure.     

Formation of interfacial voids in cast and micro-grained γ′-Ni3Al during high temperature oxidation

Specimens of cast and micro-grained γ′-Ni₃Al, which were produced with vacuum casting and unbalanced magnetron sputter deposition, respectively, were isothermally oxidised in air at 1473 K for different periods of time. The formation of interfacial voids at the alloy/oxide interface was observed with SEM, which indicated that there were more interfacial voids formed in the cast Ni₃Al than in the micro-grained alloy under the same oxidation conditions. A phenomenological equation describing the fraction of the void projected areas was established, in which the impingement and coalescence between voids during their growth was taken into consideration. It was elucidated that low vacancy density in the micro-grained Ni₃Al due to the high creep, re-crystallisation and the enhanced Al diffusion reduced the void percentage. Also, it was confirmed that aluminium evaporation, perhaps supplemented by surface diffusion, supplied most Al to the oxide scales formed above the interfacial voids.     

Mechanical properties and oxidation behaviour of two-phase iron aluminium alloys with Zr(Fe,Al)2 Laves phase or Zr(Fe,Al)12 τ1 phase

Two-phase Fe-rich Fe–Al–Zr alloys have been prepared consisting of binary Fe–Al with a very low solubility for Zr and the ternary Laves phase Zr(Fe,Al)₂ or τ₁ phase Zr(Fe,Al)₁₂. Yield stress, flexural fracture strain, and oxidation behaviour of these alloys have been studied in the temperature range between room temperature and 1200 °C. Both the Laves phase and the τ₁ phase act as strengthening phases increasing significantly the yield stress as well as the brittle-to-ductile transition temperature. Alloys containing disordered A2+ ordered D0₃ Fe–Al show strongly increased yield stresses compared to alloys with only A2 or D0₃ Fe–Al. The binary and ternary alloys with about 40at.% Al and 0 or 0.8at.% Zr show the effect of vacancy hardening at low temperatures which can be eliminated by heat treatments at 400 °C. At higher Zr contents this effect is lost and instead an increase of low-temperature strength is observed after the heat treatment. The increase of the high-temperature yield strength of Fe-40at.% Al by adding Zr is much stronger than by other ternary additions such as Ti, Nb, or Mo. Tests on the oxidation resistance at temperatures up to 1200 °C indicate a detrimental effect of Zr already for additions of 0.1at.%.     

Deformation behaviour and oxidation resistance of single-phase and two-phase L21-ordered Fe–Al–Ti alloys

Single-phase Fe–Al–Ti alloys with the Heusler-type L2₁ structure and two-phase L2₁ Fe–Al–Ti alloys with MgZn₂-type Laves phase or Mn₂₃Th₆-type τ₂ phase precipitates were studied with respect to hardness at room temperature, compressive 0.2% yield stress at 20–1100 °C, brittle-to-ductile transition temperature (BDTT), creep resistance at 800 and 1000 °C and oxidation resistance at 20–1000 °C. At high temperatures the L2₁ Fe–Al–Ti alloys show considerable strength and creep resistance which are superior to other iron aluminide alloys. Alloys with not too high Ti and Al contents exhibit a yield stress anomaly with a maximum at temperatures as high as 750 °C. BDTT ranges between 675 and 900 °C. Oxidation at 900 °C is controlled by parabolic scale growth.     

Phase equilibria in the Fe–Al–Mo system – Part I: Stability of the Laves phase Fe2Mo and isothermal section at 800 °C

Phase equilibria in the Fe–Al–Mo system were experimentally determined at 800 °C. From metallography, X-ray diffraction and electron probe microanalysis on equilibrated alloys and diffusion couples a complete isothermal section has been established. It is shown that the Laves phase Fe₂Mo is a stable phase. The phase Al₄Mo, which only becomes stable above 942 °C in the binary system, is the only ternary compound found at 800 °C. For all binary phases the solid solubility ranges for the third component have been established. The D0₃/B2 and B2/A2 transition temperatures have been determined for a selected alloy by differential thermal analysis and transmission electron microscopy. The results confirm that the D0₃/B2 transition temperature substantially increases by the addition of Mo, while the B2/A2 transition temperature is about that for a binary alloy with the same Al content.     

Strengthening of iron aluminide alloys by atomic ordering and Laves phase precipitation for high-temperature applications

The ternary system Fe–Al–Ta allows the formation of the hard and brittle ternary Laves phase Ta(Fe_0.5+x,Al_0.5−x)₂ with hexagonal C14 structure. The present study concentrates on Fe–Al–Ta alloys with small Ta contents between 2 and 6 at.% and various Al contents between 0 and 45 at.%. The phase equilibria in the ternary Fe–Al–Ta system at 1000 °C are studied experimentally for determination of the solubility limits of Ta in iron aluminide matrices and types of phases and structures which may occur at high temperatures. It is observed that small amounts of Laves phase together with atomic ordering increase the yield stress and affect ductility in a complex way.     

Polymorphism of PrIr2Si2 – In situ XRPD experiments and theoretical calculations

We have confirmed polymorphism of PrIr₂Si₂ and performed the in situ high-temperature X-ray powder diffraction (XRPD) experiments focused on the dynamics of the crystallographic phase transition from the high-temperature CaBe₂Ge₂-type phase to the low-temperature ThCr₂Si₂ phase above 250 °C. A double-exponential time evolution of this phase transformation has been observed at 325 °C. We have also performed density functional calculations to analyze why the low-temperature crystallographic modification becomes the ground state crystal structure. The important role of the c/a ratio in this process can be argued from results of calculations.